383 compression ratios

[sabre67]

Hi all I was looking compression ratios on My Mopar.com for a 1966 383 engines they say the 2bbl engine has a cr of 9.2:1 and the 4bbl is 10.0:1 cr. bore and stroke is the same on both engines. Where do they make up the higher cr , pistons,heads, cams? Thanks for any help on this question

 

[dla4567]

Pistons were lower.

 

[66Satellite47]

Piston height was what use to change compression ratio. Be aware the true CR was probably quite a bit lower than advertised, at .5 less. You don't know what the CR is until you measure all the parts.

 

[727]

2bbl pistons were dished

 

[dla4567]

2bbl pistons were dished

I’ve also had some flat tops that sat pretty low.

 

[ir3333]

The rating was usually much higher than the actual compression ratio.
The factory rated them at 10.5 so you could build them to 10.5 and be NHRA legal.
If you cc and calculate an early 10.5:1 340 you will get about 9.5:1.A later 8.5:1 340 was in the 7's.
I have never done the math on a Road Runner 383 but have read they are somewhere in the 8's.The large cc 440 heads and low pistons were the cause and that's why they really ran good once you installed 4.10's.Lower compression engines need to rev higher to make power.

 

[727]

I have seen later pistons sitting lower in the hole. Not in 1966 blocks though.

 

[66Satellite47]

My '70 383 2 BBL pistons were flat top. The FSM says they were 8.7/1 CR. MY'66 FSM says 2 BBL was 9.2 CR. As previously said true CR was lower. I CC'd dozens of BB heads, they all were much larger than an NHRA low limit. To know what the CR is you need to actually measure the parts.